IMO starting with "first of all we were there first" kind of sets the tone for the whole message. Here's how I would have written the same message:

Hi! Lotus has a great product. We should know, as they reached out to us for inspiration before. Our team has prior experience building billing engines, so we have a very good understanding of the space.

From a product perspective, our coverage is more extensive. We provide more advanced coupons, usage-based components and billing use-cases. For example, we offer credit notes and refunds, small examples that are not listed in their API. Our product covers more billing edge-cases and deeper features.

While we don't know how many customers Lotus has in production, our top user is invoicing 5M customers a month!

Overall, we believe at this point that our product is more advanced. We wish them luck though, the more products in this space the better.